Masonry heaters have been used at least since Roman times to provide radiant heat from a mass of masonry. A conventional masonry heater includes a firebox and a series of maze like channels through which the flue gases escape to the chimney. The firebox and channels have walls preferably made of refractory material which absorbs heat energy from the burning fuel and combustion gases. The primary advantage of a masonry heater is that a relatively small amount of fuel may be burned for a short period of time and the mass of the masonry heater absorbs a significant portion of the heat energy from such a fire. The masonry mass thereafter radiates heat to the surrounding room over a long period of time.
A contra-flow masonry heater conventionally has a central firebox with air intake and upper flue outlet. Vertical contra-flow heat exchange channels communicate with the flue outlet at the top of the firebox and direct the combustion gases downward in the channels to be exhausted through an exhaust port at a lower level. A contra-flow masonry heater is preferred since a larger portion of the heat energy may be absorbed by the masonry mass. The heat of combustion is high enough to avoid the buildup of creasole and other combustible products in the channels.
In Europe, masonry heaters have been conventionally built where a wood burning fireplace is desired. The size of the masonry heaters is relatively large and they are often located in the center of a home to provide radiant heating throughout the dwelling.
In North America, conventionally an open hearth fireplace has been used, however, in recent years due to the inefficiencies of such fireplaces, their use has been discouraged. A conventional fireplace is very inefficient in that it draws room air for combustion and although radiant heat is felt in the immediate vicinity of the fireplace, in total the heat output of many fireplaces is negative. In other words, the draft caused by the operation of the fireplace exhausts warm air from the dwelling and draws in cold air through doorways, windows and other openings of the dwelling. In modern home construction, insulation and vapor barriers are continuously provided to substantially increase the heating efficiency of the dwelling. A fireplace in such a dwelling represents a major source of heat loss, however, since many members of the purchasing public still desire a wood burning fireplace in their homes, the level of energy efficiency is less than optimal in modern construction. When a gas burning insert is placed in a conventional fireplace, the heat efficiency is not substantially higher than when wood is burned in a conventional fireplace. As a result, such gas inserts are largely decorative and are not a practical source of heat for a dwelling despite the quantity of gas burned in such applications.
Attempts have been made to combine the efficiency of a masonry heater to replace a conventional fireplace with the addition of a gas burning insert. Such attempts have been unsuccessful to date. Due to the contra-flow design there is a dangerous potential for non combusted gas to accumulate in the upper regions of the contra-flow masonry heater. As a result, such gas fire contra-flow heaters must be operated with the doors open and may not include a chimney damper. Inefficient burning of the gas fuel results, however the problem of accumulation of dangerous gases is avoided. A significant danger still is presented by such an application since the home owner may forget to leave the doors open or may instal a damper without knowing the inherent dangers of such a modification.
The design of conventional masonry heaters also often requires that expansion gaskets be installed between various parts of the firebox and heat exchange channels due to the differential in temperature during operation and relative expansion and contraction. Gaskets and cracks in the masonry heater may be acceptable in a wood burning application, however the danger of carbon monoxide poisoning or explosive gases is present when used in conjunction with a gas burner. Another disadvantage of conventional masonry heaters is that they are of a size which requires the removal of an existing fireplace to install a masonry heater. This is not a particular difficulty with the construction of a new home, however the market potential of masonry heaters is significantly restricted by its inability to be adapted to retro fit in a conventional fireplace.